The majority of conventional commodity polymers such as polyethylene, polystyrene and polyamide are inherently insulators due to their lack of intrinsic charge carriers. When it is required, the electrical conductivity of polymers can be increased by incorporating conductive additives such as carbon black and metal particles or conventional antistat agents.
The addition of conductive additives to polymers has expanded the application of these polymers to fields where it is desirable for the product to have some electrical conductivity. One example involves the use of electrically biasable polyurethane transfer rolls or webs, which are used in electrostatographic copying systems or apparati to transfer images from an electrostatographic element such as a photoconductor, to a final support material or receiver such as a web or sheet of paper.
The process of transferring toner material from the electrostatographic element or photoconductor to the receiving sheet or copy sheet, is realized at a transfer station. In a conventional transfer station, transfer is commonly achieved by applying electrostatic force fields in a transfer nip sufficient to overcome the forces that hold the toner particles to their original support surface on the photo-receptive member or photoconductor. These electrostatic force fields operate to attract and transfer the toner particles over and onto the copy sheet or other supporting second surface.
A biasable transfer member, such as a biasable transfer roll is used to control the forces acting on the toner during the transfer process enabling the toner to be transferred from the photoconductor to the final support material.
In order to achieve optimal image transfer, the resistivity of such materials have to be controlled to a critical range and, at the same time, the resistivity has to be relatively insensitive to moisture variations so that the resistivity of the materials remains relatively constant within the ranges required for optimal image transfer.
It has been found that the most favorable volume resistivity of the polyurethane transfer member should be between 1.0×106 and 5.0×1011 ohm cm in order to optimize the toner image transfer from the surface of the photoconductor to the final support surface.
U.S. Pat. No. 3,959,574 describes elastomeric polyurethane transfer members such as rolls and belts having ionic additives to control the resistivity. The effectiveness of the additives for reducing the resistivity of the elastomers according to the patent is achieved if the additives are soluble or dispersible in the elastomeric polyurethane. However, over time, the ionic conductivity control additives migrate out depleting ions and increasing the resistivity of the polyurethane.
Chen et al, in U.S. Pat. Nos. 4,729,925 and 4,762,941 disclose a polyurethane elastomer in which certain polyol conductivity-control agents formed from certain salts complexed with particular polyester diols such as for example, bis[oxydiethylenebis(polycaprolactone)yl]5-sulfo-1,3-benzenedicarboxylate, methyltriphenylphosphonium salt, are copolymerized with certain polyisocyanate prepolymers and other polyols normally used to make polyurethanes to yield elastomers with resistivity that can be maintained between 1.0×109 and 1.0×1011 ohm cm. According to this patent, the conductivity control agent is an integral part of the polymer and therefore not subject to being leached out as described in the prior case in which the conductivity control agent is included as an additive. The polyurethane elastomers of this patent however, are still moisture sensitive. For example, curve 2 in FIG. 2 of U.S. Pat. No. 4,729,925, indicates that the volume resistivity of the conductive polyurethane elastomers of Example 15 decrease by a factor of about 6.5 when the relative humidity changed from 25% to about 85%.
Wilson et al, in U.S. Pat. No. 5,212,032, disclose, as coating materials for biasable transfer members, certain elastomeric polyurethanes containing, as conductivity control agents for controlling the resistivity of the elastomeric coating and hence that of the biasable transfer member to a range from about 1.0×107 to about 5.0×1010 ohm cm, certain ionizable ferric halides selected from the group consisting of ferric fluoride, ferric chloride and ferric bromide complexed with ethylene glycol or an oligoethylene glycol selected from the group consisting of di-, tri-, and tetraethylene glycol.
However, although the polyurethane materials of Chen et al and Wilson et al possess volume resistivity in a range compatible with or critical to optimal toner image transfer, they are deficient in that they both exhibit or possess relatively short electrical lives. That is, after certain hours of continuous use in an electrostatographic copying device, a biasable transfer member utilizing a polyurethane material of either Chen et al or Wilson et al must be removed from the copying device or machine and replaced with a new biasable transfer member because the original biasable transfer member no longer is capable of transferring a complete toner image from the photoconductor to the final support material (e.g. a sheet of paper). This is believed to be due to the following phenomena. Under normal operating conditions, it is necessary in order to achieve optimal image transfer to maintain a relatively constant current flow of less than about 30 micro amps in the nip area between the transfer roll surface, the transfer material and the photoconductive surface from which a developed image is to be transferred. For this condition to exist, the resistivity of the polyurethane material must be within critical values, i.e., from about 1.0×106 to about 5.0×1011 ohm cm, as previously mentioned, and must be relatively constant under normally anticipated extremes of operating conditions. The electrical life, and hence the functional life of the biasable transfer member (i.e., the working life of the biasable transfer member) is directly related to the maintenance of this constant controlled resistivity region. That is, the electrical life of the biasable transfer member is largely determined by the stability of the output current and/or voltage versus time. (Bias roll power supplies are generally constant current or constant voltage devices with upper current or voltage limits, which respond to changes in the resistivity of the biasable, roll material, i.e., the polyurethane). Thus, as used herein, the term “electrical life” refers to a controlled, i.e., constant resistivity with time under an applied electrical field. Changes in the resistivity of the polyurethane material versus time are reflected in voltage demands required to maintain the constant current output of the material of which the device is made. As transfer current flows through the biased transfer member or roll, however, over time the ionic conductivity control additives in the polyurethane materials used in the biasable transfer roll migrate, depleting ions and increasing the resistivity of the material causing the bias voltage to increase while maintaining a constant transfer current. Eventually, substantially all of the ions are depleted and the upper voltage limit is reached beyond which point the efficient transfer of toner can no longer take place resulting in incomplete toner transfer causing undesirable side effects such as mottle or no toner transfer at all. Thus, the material used in the fabrication of a typical biasable transfer member (e.g., a biasable transfer roll) has an intrinsic electrical life directly related to the ionic depletion of the conductivity control agent in the polyurethane material. Stated another way, the problem associated with bias roll transfer systems is that the electrical life of the bias transfer member is inversely proportional to the transfer current therethrough.
Vreeland et al, in U.S. Pat. No. 5,571,457, disclose as coating materials for biasable transfer members, certain elastomeric polyurethanes containing, as conductivity control agent, a blend composed of a dicarboxylate salt of Chen et al with a ferric halide/ethylene glycol or oligoethylene glycol complex of Wilson et al in various molar ratios. According to this patent, the incorporation of the blend into a polyurethane material provides a resistivity to the polymeric material of from about 1.0×106 to about 5.0×1011 ohm cm and in addition to that, improves or extends the electrical life of the polyurethane material beyond the electrical life of either of the polyurethane materials of Chen et al or Wilson et al. However, this patent does not mention any correlation between the electrical life and the environment in which the test was conducted.
It would be important in the art for a biasable transfer member to not only have a controlled or adjusted specific resistivity range and a constant resistivity with time under an applied electrical field but also that the resistivity and the resistivity versus time both be insensitive to widely varying changes in absolute humidity encountered in normal operating conditions such that the resistivity remains relatively constant within the range required for optimal image transfer. The present invention provides a biasable transfer member and methods for making same which has an improved or extended electrical life in dry environment compared with materials described in prior art.